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A common spamming technique of sending unwanted e-mail pitches as image attachments rather than text is on the decline, as spammers continue adapting their methods for sneaking past e-mail filters.

This week, security company McAfee Inc. revised its top 10 predictions for the year to account for the reversal. Another company, IronPort Systems Inc., said image spam now accounts for about 20 percent of all spam, down from 33 percent two months ago.

”It’s been two years since image spam has burst onto the scene. Anti-spam technologies are starting to react to that,” said David Mayer, a product manager at IronPort. ”We’ve seen other techniques and technologies rise up to make up” for the decline.

He said spammers are now placing those images on free photo-sharing sites – the ones people use to send vacation photos to friends and family – and embedding links to those images in their junk messages. These are difficult for spam filters to block because the same sites are used for legitimate photos as well.

And on Wednesday, IronPort said it saw the first spam attack using PDF attachments, showing spammers’ increased sophistication and willingness to diversify their portfolio.

Images are popular among spammers because filters have no easy way of knowing whether a graphical file contains an innocent photograph of a friend’s birthday party or embedded text pitching Viagra or a company’s stock.

Filters initially applied a mathematical formula to known spam images, generating a unique signature that software can use to flag junk. But then spammers circulated tools to automatically vary images ever so slightly – a change in color here, a slightly larger border there – changing the signature and helping spam escape detection.

But filters have now gotten better at scanning the contents of the attachments, leading spammers to link instead to images elsewhere. Within weeks, Mayer said, use of that technique rose from almost nothing to about 5 percent of all spam.

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